Appending array while using recursive loop in Ruby

Question

I am working on Collatz Conjecture in Ruby and stop at n = 1. This is the code that I have so far:

def collatz(n)
  arr = []
  if n == 1
    arr << n
    return arr
  elsif n % 2 == 0
    n = n / 2
    arr << n
    collatz(n)
  else
    n = (3 * n) + 1
    arr << n
    collatz(n)
  end

end

I want the code to return an array of all n values throughout the loop. For example, if I do collatz(10), I wish it to display [5,16,8,4,2,1]

Right now it displays [1] for any input value. This is because after the first loop, it calls collatz(n), and the first line says arr= [], thereby resetting my array each loop.

How can I continue the method loop while appending my array?

Lukas Baliak · Accepted Answer · 2016-06-30 22:19:39Z

1

You can use this code.

If n is 1 return it like Array. If n % 2 == 0 use n / 2 else use (3 * n) + 1. if n is now 1 return it like Array else define new Array with n value and call yourself with new n.

def collatz(n)
  return [n] if n == 1
  n = n % 2 == 0 ? n / 2 : (3 * n) + 1
  n == 1 ? [n] : [n] + send(__method__, n)
end

output

p collatz(10)
# => [5, 16, 8, 4, 2, 1]

I hope this helps

edited Jun 30, 2016 at 22:19

answered Jun 30, 2016 at 22:10

Lukas Baliak

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Comments

Sergio Belevskij · Accepted Answer · 2016-07-01 02:12:58Z

0

add array to params as exampled:

def collatz(arr, n)
  if n == 1
    arr << n
    return arr
  elsif n % 2 == 0
    n = n / 2
    arr << n
    collatz(arr, n)
  else
    n = (3 * n) + 1
    arr << n
    collatz(arr, n)
  end
end

puts "#{collatz([], 10).join(', ')}"

result will the same as you wish:

[5, 16, 8, 4, 2, 1, 1]

But remember: recurse is a evil!

and within recurse:

def collatz(n)
  arr = []
  loop do
    n = n % 2 == 0 ? n / 2 : (3 * n) + 1
    arr << n
    break if n == 1
  end
  arr
end

puts "#{collatz(10).join(', ')}"

edited Jul 1, 2016 at 2:12

answered Jun 30, 2016 at 18:39

Sergio Belevskij

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2 Comments

Lukas Baliak Over a year ago

Why is recurse evil?

Sergio Belevskij Over a year ago

recurse is evil because it limit your abilities with maximum call stack limitations. It (callstack limit) depends on your language and computer, but it not infinite.

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